Virtually all modern wind energy installations are de-signed for a variable rotation speed. This means that the wind rotor, which generally drives the generator via a transmission, can be operated at a different speed, depending on the wind conditions. To this end, a capability is provided to vary the pitch angles of the rotor blades of the wind rotor. Varying the pitch angle varies the wind power that the wind rotor extracts from the wind. The torque control unit correspondingly varies the torque of the generator, and therefore the emitted electrical power. A conventional closed-loop control system generally provides for the pitch control unit and the torque control unit to be connected to a superordinate operating point module, which determines nominal value presets for the pitch and torque control units, and applies them thereto.
The control device can be designed such that the pitch control unit and the torque control unit are independent of one another (U.S. Pat. No. 6,137,187). However, it is also possible for the two control units to be linked to one another (DE 10 2005 029 000), in such a way that the linking makes it possible to achieve a significant improvement in transitional behavior between partial-load operation and full-load operation of the wind energy installation.
When grid disturbances occur during operation, in particular brief voltage dips as a result of a short, then variable rotation-speed wind installations can also be affected by them. Conventionally, the wind energy installation is disconnected from the grid, as a result of which less power is available in the grid. This is counterproductive in the event of a short. It is therefore desirable to keep the wind energy installation connected to the grid, at least during short voltage dips, thus allowing power to be fed into the grid again from the wind energy installation as quickly as possible at the end of the voltage dip. This aspect of the wind energy installation still being connected to the grid throughout the duration of the voltage dip is referred to as “low voltage ride through”.
Because of the rapid changes which occur in the electrical grid parameter when the grid collapses, corresponding, highly dynamic effects occur on the wind energy installations and their drive train, resulting in oscillations. These oscillations, which occur at the start of the grid dip, are in practice excited again at the end of the grid dip, that is to say when the voltage returns. Torque peaks can occur in this case, which are more than twice the rated torque. There is therefore a risk of the drive train of the wind energy installation fracturing, and a risk of damage to the surrounding area. One known remedy is to appropriately derate the mechanical drive train. However, this has the disadvantage that the wind energy installation production costs are considerably increased.